Optical reformatters tend to be implemented for receiving input images and light beams and producing reshaped output images and light beams which are better suited for measurement by an optical system such as an optical spectrograph or a detector or detector array, or for further processing by a light processing system. Specifically, optical reformatters are useful for preparing and configuring light that passes to an optical spectrograph from the output of a light source such as an optical fiber, a bundle of optical fibers, a telescope, an image relay, or a physical aperture such as an input slit.
By way of background, conventional optical spectrographs include a small input aperture, typically a slit. The input aperture could alternatively be a circular pinhole, an optical fiber, or other input means; however, for the sake of brevity, the input aperture will hereinafter be referred to as a slit. An input light may be a converging or diverging beam of light projected towards the slit, or may be some other light source placed such that a portion of the light passes through the slit. In a typical optical spectrograph, light passing through the slit is projected onto a lens or mirror which collimates the light to form a beam of substantially parallel light rays. In a typical optical spectrograph, a dispersive element, such as, a prism, a transmission grating, or reflection grating, bends the collimated beams by differing amounts, depending on the wavelength of the light, thereby producing a spectrally dispersed light beam. Often, a camera lens or mirror brings these spectrally dispersed beams into focus on an array detector, such as a charge-coupled device (CCD) detector, or some other single element or multi-element detector located at the final focal plane, and which may measure the focused spectrum and record the light intensities of the various wavelengths.
In a typical optical spectrograph, the collimating lens (or mirror) and the camera lens (or mirror) act as an image relay, to create images of the light passing through the slit on the detector, such as a CCD detector, with the images displaced laterally depending on the wavelength of the light. The spectral resolution of an optical spectrograph, a quantitative description of its ability to detect and measure narrow spectral features such as absorption or emission lines, can be dependent upon various characteristics of the spectrograph. Such characteristics may include the dispersing element, for example the prism, transmission grating, or reflection grating; the focal lengths of the collimating lens (or mirror) and camera lens (or mirror); and the width of the slit along the dispersive axis. For a particular disperser and camera lens, the resolution of the spectrograph can be increased by narrowing the width of the input slit, which causes each image of the light passing through the slit (depending on the wavelength of the light) and onto a detector, to subtend a smaller section of the detector, allowing adjacent spectral elements to be more easily distinguished from each other.
By narrowing the width of the input slit, less light passes therethrough, which can reduce the quality of any measurements due to a reduction in the signal-to-noise ratio. In some applications, such as astronomical spectroscopy, high-speed biomedical spectroscopy, high-resolution spectroscopy, or Raman spectroscopy, this loss of efficiency can be a limiting factor in the performance of the optical spectrograph. A device which increases the amount of light that can pass through the slit by compressing an image of an input beam of light along the dispersive axis (i.e. horizontally), while substantially maintaining light intensity or flux density, would be advantageous in the field of optical spectrography even if the spot image is compressed along the dispersive axis at the expense of expansion along a perpendicular axis (i.e. vertically).
A person of skill will understand that the terms “horizontal”, “vertical” and other such terms used throughout this description, such as, “above” and “below”, are used for the sake of explaining various embodiments of the invention, and that such terms are not intended to be limiting of the present invention.
A person of skill will also understand that while the term component is usually used to refer to a specific item such as a lens or a mirror, and the term element is usually used to refer to a group of components that share a common functional purpose, it is also possible to have an element made up of a single component, or a single component which functions as multiple elements. For example, in the case of an optical component with multiple reflective or refractive surfaces such as a lens with a reflective coating, the lens could have the function of one element and the reflective coating could have the function of a different element. Similarly, a curved mirror could both redirect a light beam and change the divergence of the light beam, thereby providing the function of multiple elements in the same component.
A person of skill will also understand that the focused image produced by focusing a collimated beam may be referred to as a spot or a spot image, and that a light source does not need to be a focused spot image in order to be collimated. An image refers to the light field spatial distribution at the focal plane of a lens or mirror wherein the wavefront concavity changes direction, while image-space refers to any space in the light field where the wavefronts are substantially not planar. A pupil refers to a lateral cross section of a light field wherein the wavefronts are substantially planar, and pupil-space therefore refers to any location where the wavefronts are substantially planar.
Optical reformatters can be useful to receive an input beam and/or input image and produce output beams and/or output images that are better matched to spectrometer input slits. An optical slicer is one type of optical reformatter in which portions of the beam or image are divided up and redirected or repositioned.
An optical slicer comprising transparent prisms and plates to slice an input beam can have deficiencies because it may produce a reformatted image at a slit that is tilted along the optical axis, and additionally the slicing of an optical beam can occur along the hypotenuse of a 45° prism, which can result in focal point degradation due to different sections of the sliced image being located at different focal positions. The performance of such slicers can also depend on the absorption coefficient and index of refraction of the prism material used (which are both wavelength dependent). These deficiencies can limit the use of such slicers in broadband optical devices.
There also exists other optical slicers which are image slicers, such as the Bowen-Walraven slicer or optical fiber spot-to-line converters, which operate entirely in the image space. Some such image slicers generally do not preserve the spatial image information and are therefore unable to resolve spectral information from different portions of a source image independently. These reformatters are also challenging to implement in a commercially feasible way, can be large in size, and can result in reduced or inefficient implementation of a variety of systems. These slicers often produce multiple copies of the slit image which can result in wasted space on the detector due to gaps between the slices at the final focal plane, which may add noise to the signal and thus decrease the quality of the output data, limit the number of spectra (or spectral orders) that can fit on the detector, and reduce the efficiency of the detector readout because of the spectrum being spread over a larger detector area. Optical slicers using optical fiber bundles to allow the extended (often round) image of an input source to be formed into a narrow slit image can also cause degradation of the output f-ratio and the total performance to be inefficient. Existing slicer devices almost uniformly suffer this decreased efficiency and output f-ratio, which is a clear limitation of slicer design and implementation. Also, optical fiber bundles tend to be inefficient for light collection due to gaps between the individual fibers and space taken up by the individual fiber claddings.
More recently, new pupil reformatter designs, and the use of pupil reformatters to improve the spectral resolution of a spectrograph, have been disclosed. These slicer-based reformatters operate entirely in pupil space, slicing and then anamorphically expanding a collimated beam. This approach is useful when spatial image information needs to be preserved, such as with push-broom hyperspectral imaging, multi-fiber inputs, etc., but pupil beam divergence can be problematic with larger input sources, and optical system complexity increases with an increase in the number of slices created.
The present invention differs from existing reformatter designs in that it operates partially in pupil space and partially in image space. As such, it is referred to throughout this application as a hybrid image-pupil optical reformatter and embodiments of the present invention can be described as a hybrid slicer or hybrid reformatter, which operates partially in pupil space and partially in image space. This approach has advantages over traditional optical slicers including in instances when a larger number of slices are desired since operating a reformatter partially in pupil space and partially in image space, as disclosed in this invention, tends to be characterized by back-and-forth optical beam paths through a collimator which limits the beam from spreading out. In embodiments of the present invention, larger slicing factors may be achieved with fewer components, reduced beam divergence losses, and less demanding alignment tolerances, and the number of slices tends to be relatively independent of the optical complexity, with a preferred number of slices being approximately equal to the ratio of the input beam width to the output beam width. Embodiments of the present design also tend to more easily handle larger input spot sizes and/or faster input beams (small f-ratios) than traditional optical slicers.
The pupil beam in the present invention tends to get narrower without getting taller, and the pupil slices disclosed in embodiments of the present invention tend to overlap. This is in contrast to most pupil reformatters in which the pupil beam gets both narrower and taller, and the pupil slices generally do not overlap. Further, many other optical reformatters use ‘explicit’ expansion as part of the reformatting while the expansion is ‘implicit’ in some embodiments disclosed in the present invention.